1. Field of the Invention
The present invention relates generally to video encoders, and in particular, to an apparatus for generating VBI data coding waveforms.
2. Background Art
An important component in a video play-back system is a video encoder. The design of a video encoder is a complex endeavor that requires careful consideration of different data transmission requirements and data waveform characteristics. Typically, a video encoder should be able to handle two common color television standards: National Television Standards Committee (NTSC) and Phase Alternating Line (PAL). In addition, the video encoder should be able to handle Vertical Blanking Interval (VBI) data encoding for applications such as close-captioned programming and teletext data transmission. The NTSC, PAL, and VBI data encoding schemes are described in greater detail hereinafter.
Unfortunately, the data transmission requirements, methods, and data waveform characteristics for VBI data encoding schemes are different from the data requirements, methods, and data waveform characteristics for NTSC and PAL standards, leading to increased complexity and cost in the design and implementation of video encoders.
An important component in VBI data encoders is the waveform shaping circuit. Conventional waveform shaping circuits use a digital filter and a baud rate synchronizer that is driven by a system clock. Unfortunately, this conventional approach to waveform shaping is difficult to apply to VBI data codings because (1) baud rate types are numerous, and (2) the system clock has limitations.
An alternative approach is to utilize direct waveform synthesis for waveform shaping. Unfortunately, in order to satisfy bandlimit and minimum inter-symbol interference (ISI) requirements, a unit pulse (e.g., a raised cosine pulse with 4T duration) is utilized by the conventional approach. This unit pulse requires very large look-up tables for the direct synthesis of the waveforms. In addition, even with a large look-up table, further computations are needed after values are read from the table to account for the overlap of the unit pulse waveform.
In summary, conventional video encoders suffer from the following disadvantages. First, separate circuits are needed to convert a digital video stream into a standard NTSC or PAL signal, and additional circuits are needed to handle the VBI data transmission requirements, thereby increasing circuit complexity and cost. Second, conventional VBI data encoders require a very large look-up table for waveform synthesis to reduce inter-symbol interference (ISI) to acceptable levels. Since space is at a premium, the large area needed by the look-up table is undesirable. Third, with a large look-up table, additional circuits are needed for computations, such as addition and multiplication, to account for the overlap of the waveforms.
Accordingly, there remains a need for a method and apparatus for generating VBI data coding waveforms that overcomes the disadvantages set forth previously.
It is an object of the present invention to provide a method and apparatus for generating VBI data coding waveforms that reduce inter-symbol interference (ISI).
It is another object of the present invention to provide a method and apparatus for generating VBI data coding waveforms that reduce the size of the waveform look-up table.
It is another object of the present invention to provide a method and apparatus for generating VBI data coding waveforms that obviates the need for further computations after the values are read from the look-up table.
It is yet another object of the present invention to provide a method and apparatus for generating VBI data coding waveforms that utilizes the waveform look-up table associated with color burst processing to generate VBI data coding waveforms.
In order to accomplish the objects of the present invention, an apparatus and method for generating VBI data coding waveforms are provided. The present invention utilizes a single look-up table that stores sampled values of sine waves in direct waveform synthesis for both color burst processing and VBI data coding processing. Furthermore, the present invention utilizes a 2T duration pulse as a signalling waveform in direct synthesis to overcome the problem of overlap for conventional unit pulse durations.
The apparatus and method for generating VBI data coding waveforms according to the present invention includes a VBI coding waveform generating circuit that is provided in a video encoder for generating VBI data coding waveforms and color burst waveforms. This circuit has a first selection circuit that selects either a color burst phase data signal or a VBI phase data signal as an address for a waveform look-up table based on a color burst period signal. A second selection circuit is provided for receiving a color burst scalar signal, a VBI scalar signal, and the color burst period signal, and for generating either the color burst scalar signal or the VBI scalar signal based on the color burst period signal. The waveform look-up table is coupled to the first selection circuit and receives the address and generates a point of the waveform corresponding to the address. A multiplier is coupled to the waveform look-up table and the second selection circuit and multiplies the output of the waveform look-up table with the output of the second selection circuit and to obtain a scaled output.
In an alternative embodiment, a simplification circuit for reducing the size of the waveform look-up table is coupled to the look-up table and the multiplier. The simplification circuit includes a sign determination circuit, coupled to the look-up table and the multiplier, for receiving a value from the look-up table and for multiplying the value with either a one or a negative one based on a sign control signal. The simplification circuit further includes a third selection circuit, coupled to the sign determination circuit, for receiving a color burst sign control signal, a VBI sign control signal, and a color burst period signal, and for generating either the color burst sign control signal or the VBI sign control signal based on the color burst period signal.